Fig. 10.
The principal difference between the continuous-and alternating-current dynamo, is in the number of magnets used. Most of the former have only four magnets, while the latter have frequently as many as thirty-two. Current not commutated. In reality, as I have shown, these are all alternating-current dynamos, only that in the so-called continuous-current ones, the current is commutated, whereas in the others it is not, but is used as it is produced. In the principal alternating-current dynamos, a number of small magnets, usually sixteen, are attached to a framework directly opposite a similar number of others of the same size, the space between the ends being only about an inch or two. These are all electro-magnets, and are wound in such manner that when excited by a current, every alternate one shall have the same magnetism, as in [Fig. 10], and every opposite one a contrary magnetism.
Fig. 11.
Siemens Armature.
Intense magnetic field produced. This produces an intense magnetic field between the ends of the magnets, and in this space revolves the armature. This armature, in the Siemens dynamo, is composed of a disc having as many bobbins on the periphery as there are magnets on each side of the dynamo. As each bobbin approaches each magnet a current is induced in one direction, which is reversed when the bobbin recedes; thus an alternating current is produced, which is collected by connecting the ends to insulated rings or collars on the spindle, and having small copper brushes or rubbers in contact with them. Simplicity of Ferranti armature. In the Ferranti dynamo, the armature is quite different, and much more simple, as comparison of [Figs. 11] and [12] will show.